Topic 1.2 · Core
Dichotomous Keys
A dichotomous key turns identification into a sequence of yes-or-no decisions, and questions on this skill test two separate things: whether you can follow an existing key correctly, and whether you can build one from a set of organisms yourself.
What a dichotomous key is
A dichotomous keyidentifies organisms through a sequence of choices between two alternatives. At every stage you’re given two contrasting statements, you decide which one matches the organism in front of you, and you follow the instruction attached to that choice. Each decision narrows down the possible identities until only one remains.
Using a dichotomous key
Imagine four arthropods: Organism A has six legs and wings; Organism B has six legs and no wings; Organism C has eight legs; Organism D has many pairs of legs. A simple key to identify them could begin:
1a. Has six legs → go to 2
1b. Does not have six legs → go to 3
2a. Has wings → Organism A
2b. Does not have wings → Organism B
3a. Has eight legs → Organism C
3b. Has more than eight legs → Organism D
To use a key like this correctly:
- Begin with the first pair of statements.
- Observe the organism carefully.
- Choose the statement that matches.
- Follow the instruction attached to it.
- Continue until the organism is identified.
Start from the beginning again for each new organism. Never jump straight to a later stage, because every decision in the key depends on the choices made before it.
Constructing a dichotomous key
You can also be asked to build a key from the observable features of a set of organisms:
- Observe all the organisms carefully.
- Identify features that clearly differ between them.
- Choose one feature that separates the organisms into two groups.
- Write two contrasting statements.
- Divide each remaining group again using another identifiable feature.
- Continue until every organism can be identified.
Good choices of feature are objective and clearly observable. “Has wings / does not have wings” is a useful pair of statements. “Looks unusual / does not look unusual” is not, because it depends on opinion rather than something you can actually check. Whatever feature you pick at each step, the two alternatives need to distinguish the organisms clearly — that’s the same standard of combined, observable evidence that runs through the rest of classification, just applied one decision at a time.